We discuss delays that are measured by the RABITT technique, for which we summarize
the procedures here and refer to literature for a discussion, e.g. Ref. [1]}.
In that technique one measures photo emission spectra, where single-photon
ionization by a pulse of odd high harmonics ejects an electron in presence of a weak field at the fundamental
frequency. This generates side bands in between the odd harmonic peaks by adding a single photon of the fundamental
to one peak or, alternatively, subtracting a photon from the next higher peak.
The interference of the two pathways causes a variation of the side-band intensity
with the relative phase between harmonics and fundamental,
which in the time-domain can be interpreted as a delay in the harmonic photo-emission.
